Particulates (hereinafter also referred to as PM or soot) such as soot in exhaust gas emitted from internal combustion engines including diesel engines cause damage to the environment and human bodies, which has been a problem. Since exhaust gas contains toxic gas components such as CO, HC, and NOx, there has also been a concern for the influences of the toxic gas components on the environment and human bodies.
Thus, various filters having honeycomb structures (honeycomb filters) formed of porous ceramic such as cordierite and silicon carbide have been proposed as exhaust gas purifying apparatuses. Such honeycomb filters are connected to internal combustion engines to capture PM in exhaust gas, or to purify toxic gas components such as CO, HC, and NOx in the exhaust gas.
In addition, in order to improve fuel economy of internal combustion engines and to eliminate problems during driving attributed to the increase in pressure loss, there is a demand for a honeycomb filter in which the initial pressure loss is low or a honeycomb filter in which the increase rate of pressure loss after the accumulation of a certain amount of PM is low.
Increasing the aperture ratio is an effective means to reduce the pressure loss. Yet, increasing the aperture ratio entails a decrease in the thickness of cell partition walls. This makes it difficult to ensure the strength of the honeycomb fired body.
In the honeycomb fired body, maintaining the pressure loss at a low level and ensuring the strength are conflicting properties that are difficult to obtain simultaneously.
To solve the above problems, Patent Literature 1 discloses a honeycomb filter with an improved cell structure.
Specifically, Patent Literature 1 discloses a honeycomb structured body formed by combining a plurality of porous ceramic members with one another with an adhesive layer therebetween. Each porous ceramic member includes cells longitudinally arranged in parallel across cell partition walls, and an outer wall at the periphery of the porous ceramic member. The outer wall of the porous ceramic member is thicker than the cell partition walls. At least one cell located at the outermost periphery of the porous ceramic member is provided with a filling material at at least one corner of the cell to fill the corner.
FIG. 13 is a cross-sectional view of the porous ceramic member constituting the honeycomb structured body disclosed in Patent Literature 1 in a plane perpendicular to the longitudinal direction of the porous ceramic member. In this cross section, each square cell 221a, which is located at the outermost periphery and is partitioned by the cell partition walls extending perpendicular to an outer wall 223a of a porous ceramic member 220, is provided with a filling material in the form of a right-angled triangle at at least one corner of the cell 221a. 
In Patent Literature 1, owing to the cell structure described above, a high aperture ratio is obtained to maintain the pressure loss at a low level while the strength of the porous ceramic member is ensured, thus preventing the occurrence of damage such as cracking.